This invention relates to vertical roller mills of the type which include a rotating grinding table and grinding rollers which cooperate with the grinding table for comminuting material therebetween. More particularly, the invention relates to an improved apparatus for exerting a downward force on the grinding roller.
Prior to the present invention, roller mills of the type to which the present invention relates were known. These mills are particularly suitable for comminuting material such as coal, limestone, and cement raw meal. These roller mills include a generally horizontal grinding table rotated by a motor with at least one and preferably between two and four grinding rollers mounted at an angle to the grinding table for rotation about their own axis. The mill will include a mill body with a classifier mounted at the top. Material is supplied to the grinding table for comminution between the grinding table and the roller. A downward force is exerted on the grinding roller to crush or comminute material between the roller and the table. Air is passed through the mill and entrains the fine material and lifts the same to the classifier. The coarse fraction is returned to the mill and the fine fraction exits the mill with the vent air to a gas solids separator were product is collected. Example of this type of roller mill are shown in U.S. Pat. Nos. 4,339,080; 4,235,385; and 4,432,500.
A typical roller mill will have the grinding roller connected to a rocker arm which is pivotal relative to the mill body about a central point. Downward force is exerted on the grinding roller by exerting a force on the rocker arm tending to rotate the rocker arm and hence the grinding roller relative to the grinding table about the central pivot point. With prior practice, the means for exerting a force on the rocker arm includes a hydraulic piston-cylinder arrangement with the piston means directly connected to the rocker arm. The connection may be made through a spring element. Material between the table and roller will vary in depth and cause the grinding roller to be in a state of steady upward and downward movement. This movement will be transferred through the rocker arm to the piston-cylinder means. With prior practice, the connection between the piston means and the rocker arm is such that movement of the grinding roller and rocker arm tends to cause the piston to bind against the cylinder causing excessive wear on the piston and cylinder. In addition, the movement of the rocker arm is insufficient to cause adequate relative movement of the pivotal connection between the rocker arm and the piston so that bearings tend to have excessive wear.
The present invention may also be used in connection with any device where it is desired to exert a downward force on a member.